Oil pipelines, particularly those extending from wellheads, typically carry a mixture of oil and water, and often significant amounts of gas.
Typically it is desired to know the flow rates of the oil and water.
A large number of devices are known to the skilled person for performing flow rate measurements. It is generally accepted that no single instrument can perform all the necessary measurements for fully characterising multiphase flow, and so many known flowmeters are composed of a number of devices grouped together. Measuring pressure drop to estimate flowrates, for example, necessitates homogenization of the flow field to alleviate the problem of phase slippage, which requires either changing flow direction abruptly or thorough mixing by a highly intrusive device. In addition to measuring pressure drop, it is often necessary to determine the fraction (by volume, area or cord) that each phase occupies in the measurement pipe. Measurements made by devices used to perform this determination can be susceptible to significant errors as the determinations are either highly dependent on phase properties (e.g. in a microwave water cut meter), or on the presence of one phase with another (e.g. in a Coriolis mass meter). Also, many of these devices intrude on the flow and involve relatively expensive and/or complicated apparatus.
Furthermore, many of them only measure total liquid/fluid flow rates, further measurements and analysis being needed to derive separate oil and water flow rates.
In horizontal or near horizontal pipelines at relatively low flow rates, buoyancy forces tend to separate out the oil and water, and the flow can become stratified. The flow velocities of the oil and water can then differ with slip occurring at the interface between the oil and the water. Stratified Flow Model and Interpretation in Horizontal Wells, by B. E. Théron and T. Unwin, Society of Petroleum Engineers (SPE) paper 36560 presents a model of stratified oil-water flows in horizontal wells. A further problem with many known flow rate measuring devices is that they are incompatible with stratified liquid flows where slip occurs at the oil/water boundary.
There is a need for a relatively simple system for measuring liquid flow rates, particularly in stratified liquid flows.